The present invention relates to a method of and apparatus for producing injection-molded parts reinforced with long fibers.
It is generally known to make structural parts, such as thermoplasts reinforced with glass mats (GMT), from long fiber reinforced thermoplastic material in such a way that semi-finished products in the form of glass mats impregnated with melt are heated and shaped by a subsequent compression process. During the compression process, the mold cavity will be completely filled by the flow of the semi-finished product. The structural part is removed from the open press after a cool-down period for subsequent transfer to a post-treatment station. This type of manufacturing process is rather complicated in view of the need for successive processing steps and the required treatment stations.
It is also known to make structural parts of fiber-reinforced thermoplastics by processing plastic granules with randomly oriented fibers in a conventional screw-plunger injection molding machine. This process permits introduction of fibers of a length of only 0.2 to 0.4 mm into the structural part, so that the strength of the structural part can be improved only to a limited extent.
It is further known to make structural parts from long fiber reinforced thermoplastic granules. The granules involved here are so-called long fiber pellets which are produced by impregnating parallel glass fiber strands with a thermoplastic melt during advance through an impregnator head. The strand issuing from the impregnator head is subsequently cut to a defined pellet length between 10 and 50 mm by a pelletizer. During processing of the long fiber pellets in a screw-plunger injection molding machine, the relatively long fibers are, however, exposed in the screw cylinder of the plastifying unit to substantial shearing stress so that the major part of the fairly long fibers is sheared off or breaks. The strength of the finished injection-molded product can thus be improved also only to a limited extent.
It is also known from the Handbook, VDI-Verlag, Fortschrittsberichte VDI, Dipl.-Ing. Frank Truckenmxc3xcller, xe2x80x9cDirektverarbeitung von Endlosfasern auf Spritzgiexcex2machinenxe2x80x9d [Direct Treatment of Continuous Fibers with Injection Molding Machines], Series 3 Verfahrentechnik, No. 444, Apr. 1996, to introduce in a screw-plunger injection molding machine continuous fiber strands directly into the screw cylinder of the plastifying unit. Also in this case, the fiber strands are exposed during advance through the plastifying unit to substantial shearing stress so that the major fraction of the fibers is split in lengths that are too short.
German Pat. No. 195 23 490 discloses a two-shaft extruder with screw shafts running in same direction having a zone in which endless fibers are introduced into the screw cylinder at low pressure. Hereby, the endless fiber strands are thoroughly mixed with the melt in a continuous operation to a homogenous and gentle mixture which issues as mass strand at low pressure from the two-shaft extruder. This type of apparatus is known as compounder and effects an optimum mixture in a continuous operation. As the fiber-melt-mixture is produced continuously, this apparatus can be used for producing the long fiber granulate or long fiber pellets or for depositing the fiber-melt mixture on open mold halves via a slot die. After the fiber-melt mixture has been essentially applied on a surface of the mold half, both mold halves are pressed together and the structural part is completely shaped by a flowing process. While this process yields an optimum mixture of long fibers and melt, the further working steps of flat application of the fiber-melt-mixture upon a mold half and compressing are relatively time consuming operations to make thermoplastic parts reinforced with long fibers.
German Pat. No. DE 195 38 255 describes an extruder with two screws running in same direction for producing a continuous stream of a plastic melt which is laden with long reinforcement fibers and transferred by a subsequent injection unit, comprised of a cylinder and a piston, into the mold cavity of an injection mold at a pressure profile required for the molding process. In this apparatus, the continuous melt stream is initially conducted intermittently into an intermediate reservoir and from there into the cylinder space of the injection unit. Before entering the mold cavity, the melt stream laden with long reinforcement fibers is transported twice into the cylinder volume and out again. By reversing the flow direction twice, a great number of long fiber portions break.
FIG. 1 shows a schematic, partially sectional, illustration of a conventional arrangement for making injection-molded parts reinforced with long fibers, including a compounder 1 which has a housing 2 for accommodating two screws 3, 4 running in same direction and meshing with one another. The screws 3, 4 are operated continuously by a rotary drive 5 having a motor M. Arranged on the housing 2 is a feed hopper 6 for supply of thermoplastic granulate and a feed device 7 for introduction of a continuous long fiber strand 9 reeled off a roving 8. The long fiber strand 9 may, for example, be made of glass fiber. Basically any type of reinforcement is suitable, such as, e.g. carbon fibers, Kevlar fibers or natural fibers like hemp fibers.
The compounder 1 has an anterior screw chamber 10 which is fluidly connected via two branches 11, 12 to two pressure generating and discharge assemblies 13, 14, whereby a flow control valve 15 regulates the flow from the anterior screw chamber 10 in alternating sequence to the two pressure generating and discharge assemblies 13, 14. As shown in FIG. 1 by the partially sectional illustration of the lower one of the pressure generating and discharge assemblies 13, 14, each of the assemblies 13, 14 includes an injection plunger 16 and a hydraulic operating plunger 17 which is acted upon alternatingly via the hydraulic ports 18, 19 by a pressure fluid conveyed by a, not shown, pump for providing a predetermined pressure profile. Each of the pressure generating and discharge assemblies 13, 14 has an anterior cylinder space 20 which is fluidly connected via an injection nozzle 21 and a sprue channel 22 with a mold cavity 23 of an injection mold. The injection mold has a fixed mold mounting plate 26 for carrying a mold half 25, and a moving mold mounting plate for carrying a mold half 24, with guide rods 28 extending between the mounting plates 26, 27. The mold mounting plate 27 is moved to the opening and closing positions by a hydraulic unit 30, which is shown only partially.
At operation, the compounder 1 produces a continuous mass stream of thermoplastic melt which is permeated in a homogenous manner by a continuous fiber strand from the long fiber strand 9 drawn in via the feed device 7. The mass stream flows, controlled by the flow control valve 15, into the cylinder space 20 of the assembly 13. After the cylinder space 20 is completely filled with long fiber interspersed melt, the valve 15 is switched over to cut the connection between the anterior chamber 10 of the compounder 1 and the cylinder space 20 of the assembly 13 and to connect the anterior chamber 10 of the compounder 1 with the cylinder space 20 of the assembly 14. Subsequently, the injection plunger 16 is operated to inject the melt via the injection nozzle 21 and the sprue channel 22 into the cavity 23 between the closed mold halves 24, 25. The predetermined pressure profile in the cavity 23 for producing an injection-molded part 29 is adjusted trough appropriate admission of hydraulic pressure fluid to move the operating plunger 17.
It would be desirable and advantageous to provide an improved method and apparatus to obviate prior art shortcomings.
According to one aspect of the present invention, an apparatus for producing thermoplastic injection-molded parts reinforced with long fibers, includes a compounder having two meshing screws rotating in a same direction for continuously generating a stream of melt of thermoplastic material reinforced with long fibers; at least two piston and cylinder units alternatingly receiving melt from the compounder for injection into at least one cavity of an injection mold at a suitable pressure, wherein each said piston and cylinder unit includes a cylinder and a differential piston movably received in the cylinder and dividing the cylinder into a feed chamber and an injection chamber, wherein the piston has a greater piston surface, bounding the injection chamber, and a smaller piston surface, bounding the feed chamber, wherein the differential piston has a passageway for interconnecting the greater piston surface and the smaller piston surface to permit a flow of melt from the feed chamber into the injection chamber; and at least two non-return valves to inhibit a flow of melt from the injection chamber back to the feed chamber, with one non-return valve disposed in the passageway of the differential piston of one piston and cylinder unit, and the other non-return valve disposed in the passageway of the differential piston of the other piston and cylinder unit.
According to another aspect of the present invention, a method of producing thermoplastic injection-molded parts reinforced with long fibers, includes the steps of continuously feeding a stream of melt of thermoplastic material reinforced with long fibers to a feed space of at least two piston and cylinder units, each having a piston; and controlling a pressure upon the piston and cylinder units such that melt is able to flow from the feed space to an injection space in a cylinder of one piston and cylinder unit as a consequence of a differential pressure prevalent on opposite piston surfaces of the piston of the piston and cylinder unit to thereby fill the injection space, whereas melt in the injection space of the other piston and cylinder unit is forced into a cavity of an injection mold while prevented from returning to the feed space and while the feed space receives melt.